If you can't let go yet, don't let go. Let your mind settle the images. Maybe it'll take a day or two or three.

If after a week or so your mind is still chewing it over, I'd highly recommend seeing a counsellor for a tune up. My ex and I witnessed a very ugly domestic violence/attempted murder. Seeing a counsellor afterward was a huge help, especially later when we had to be witnesses at the perpetrator's trial and it all came up AGAIN.

You experienced a trauma. Think of it as an injury that needs to heal. It will be ok.